Is sleeping with a girl cheating?

Gabby
I'm in a debate with my friend she's been on and off with her man for a year and while they were together she had her first lesbo experience and had a girl go down on her. And she's been claiming she been nothing but good to her man but I see that as cheating?! I mean right? He doesn't know about it and it was actually with one of our friends that we party with it was just one hook up nothing after that they weren't talking like that or anything before or after. But I feel like that's cheating although she begs to differ it's not. If she was in a lesbian relationship and slept with another girl that'd be cheating so how is that not her cheating on her man? Anyways would y'all consider this cheating?